License plates carried by vehicles commonly bear an alphanumeric identification code commonly referred to as a license plate number and which is referred to herein as a license plate character set. Since each vehicle is uniquely identified by this number, it is widely used by law enforcement agencies and private concerns for the purpose of identifying vehicle ownership. Often, license plate numbers are stored in large data bases so that identification information associated with a particular number can be rapidly obtained.
Apparatus such as that described in co-pending patent application Ser. No. 859,639, filed May 5, 1986, now U.S. Pat. No. 4,817,166, entitled "Apparatus for Reading a License Plate", and having the same assignee as the present invention is capable of reading a license plate by converting the license plate number and name of the issuing state into digital form for use by a computer. Briefly, such an apparatus includes an imaging system for producing an image including the area of the license plate and processing means which scan the image and locate the region of the image which is the license plate. An extractor within the apparatus classifies the background as light or dark, and image scanning means scan the plate across a plurality of horizontal paths and senses the image intensity transitions along the scanned paths. Edge finder means utilize the sensed intensity transitions to locate the edges of the license plate characters. The edges are then traced by contour tracing means and each character is identified by identification means as the traced edges are compared to predetermined criteria contained within the database of the apparatus.
The apparatus such as aforedescribed is limited by certain factors relating to the nature of certain types of license plates or the reflectivity of the plate to light. For example, if the license plate characters are relatively thick, conventional methods of determining whether the background is light or dark may lead to erroneous background determinations if the license plate characters are relatively thick in comparison to the width of the plate. Furthermore, the heads of rivets or screws which may be partially contained within a license plate character may be responsible for an erroneous tracing of the edges of the character. Still further, glare reflected from the plate may lead to an erroneous interpretation as to the presence or location of a character edge. Yet still further, if the license plate is canted in relation to the horizontal so that the characters of the plate are canted, a proper identification of the license plate characters from the traced edges thereof is rendered difficult. Moreover, the existence of a decorative and vertically-oriented borderline adjacent the left or right edge of the plate may lead to an erroneous interpretation of the borderline as the numeral "1" or the letter "I".
Accordingly, it is an object of the present invention to provide a new and improved license plate reader and an associated method which circumvents difficulties involved in reading a license plate character set as a consequence of factors such as those described above.
Another object of the present invention is to provide such a reader and method for determining the background of the license plate with greater accuracy.
Still another object of the present invention is to provide such a reader and method for isolating the character region of the license plate with greater accuracy.
Yet another object of the present invention is to provide such a reader and method for isolating the character region of the license plate by compensating for rivets or the like which are partially contained within a plate character and by compensating for decorative borderlines positioned to the right and the left of the license plate character set.
A further object of the present invention is provide such a reader and method for isolating characters of the plate for interpretation by compensating for glare reflected from the plate.
A still further object of the present invention is to provide such a reader and method for isolating characters of the plate for interpretation by compensating for any canting of the plate and the characters thereof relative to the horizontal.
A yet still further object of the present invention is to provide an apparatus and method for identifying the state which issued the license plate with greater accuracy.
One more object of the present invention is to provide such a reader and method for interpreting characters of the license plate with greater accuracy.
Still one more object of the present invention is to provide such a reader and method for verifying the interpretation of the license plate characters with greater accuracy.
Yet one more object of the present invention is to provide such a reader and method for reading a license plate when the characters thereof are contained on two horizontal lines.
This invention resides in an improved apparatus for finding and identifying the characters of a license plate having a set of characters and a background region bordering each character, means for scanning the image and identifying a block of the image which is the image of the license plate, means for locating the edges of the license plate characters as a function of sensed image intensity changes as the license plate is scanned, means for tracing the contours of the edges of the characters, means for recognizing and identifying the characters of the license plate as a function of the traced contours, and means for recognizing and identifying the state which issued the license plate.
In one aspect of the apparatus of the present invention, the improvement comprises plate finding means associated with the image scanning means including a database containing criteria relating to the expected height of license plate characters contained within the produced image and means for scanning the character set in a columnar fashion. In this connection, the line scanning means is adapted to scan one-line-at-a-time for a predetermined distance across a portion of the license plate character set from a location situated between the top and bottom boundaries of the license plate character set portion toward the top boundary thereof and moving one-line-at-a-time across the character set portion toward the bottom boundary thereof so that the character set is scanned generally upwardly and downwardly along one vertically-oriented column having a width which is about equal to the predetermined distance. The plate finding means further includes means for measuring at least one of the average gray level intensity and the gray level variance along each line scanned by the line scanning means. Sensing means contained within the plate finding means includes means for sensing at a location adjacent the top boundary of the character set a significant decrease in the measured one of the average gray level intensity and the gray level variance between one scanned line and another scanned line as the one and another lines are scanned in sequence and identifying the one scanned line as the top edge of the character set and for sensing at a location adjacent the bottom boundary of the character set a significant decrease in the measured one of the average gray level intensity and the gray level variance between two successively-scanned lines and identifying the first of the two successively-scanned lines as the bottom edge of the character set. The plate finding means further includes means for measuring the distance between the identified top and bottom edges of the character set and for comparing the measured distance to height criteria contained within the database to determine whether the measured distance corresponds to an expected height of the character set.
In another aspect of the apparatus of the present invention, background classifier means are associated with the edge locating means for classifying the background of the plate as light or dark. The background classifier means includes scanning means for scanning the plate one-line-at-a-time across lines extending generally horizontally across the license plate character set beginning at a location situated between the top and bottom boundaries of the character set and moving one-line-at-a-time toward one of the top and bottom boundaries of the character set. The background classifier means further includes measuring means for measuring the average gray level and the gray level variance along the scanned lines and sensing means for sensing a significant decrease in the measured gray level variance from one scanned line to another scanned line as the one and another lines are scanned in sequence. Averaging means is included in the classifier means for averaging the measured average gray level of preselected ones of lines scanned previously to the scanning of another scanned line to thereby obtain an averaged gray level, and comparing means are included for comparing the average gray level of the another scanned line with the averaged gray level and classifying the background according to whether the average gray level of another scanned line exceeds or is less than the averaged gray level to thereby obtain a background determination.
In still another aspect of the apparatus of the present invention, the apparatus includes line-scanning means associated with the edge-locating means for scanning the license plate along lines extending across the license plate character set and means for composing a gray level histogram of points contained in the license plate wherein the points include points of the lines scanned by the line-scanning means. Measuring means measures the area of the license plate image and calculates a threshold gray level value which a preselected percentage of the plate is higher than. Background classifier means classifies the background as light or dark, and interpreting means interprets as part of the license plate characters the regions of the license plate which have a gray level which is at least as high as the calculated threshold level when the background is classified as dark and for interpreting as part of the license plate characters the regions of the license plate which have a gray level which is lower than the calculated threshold when the background is classified is light.
In still another aspect of the apparatus of the present invention, the apparatus includes background classifier means for classifying the background of the plate as light or dark and the edge locating means is operatively connected to the background classifier means. With the use of a Laplacian operator, the edge locating means identifies, when the background is classified as dark, a selected portion of the edges of the characters which have a selected intensity level within the range of sensed edge intensity levels and is a selected percentage of the range of the background intensity level and for identifying, when the background is classified as light, a selected portion of the edges of the characters which have a selected intensity level within the range of sensed edge intensity levels and is a selected percentage of the range below the background intensity level. The contour tracing means of the apparatus is adapted to trace the contours of the characters along the identified portions of the edges.
In yet still another aspect of the apparatus of the present invention, the apparatus includes character alignment means for defining a straight baseline and a straight top line based on the lowermost and uppermost points of the characters, and detecting means are associated with the character alignment means for detecting the presence and location of intensity transitions along each of the baseline and top line that are nearest the left edge of the license plate and for detecting the presence and location of the intensity transition along each of the baseline and top line that are nearest the right edge of the license plate. The detecting means then joins each of the intensity transition locations detected adjacent the left edge of the license plate with an imaginary first line and joins each of the intensity transition locations detected adjacent the right edge of the license plate with an imaginary second line. The character alignment means is adapted to disregard license plate information sensed to the left of the imaginary first line and to disregard license plate information sensed to the right of the imaginary second line.
In a further aspect of the apparatus of the present invention, the apparatus includes character alignment means for defining a straight baseline and straight top line and for detecting the presence and location of an intensity transition along a low path extending parallel to and positioned adjacent and below the defined straight baseline and the existence and location of an intensity transition along a high path extending parallel to and positioned adjacent and above the defined top line. The apparatus also includes means for comparing the location of the detected intensity transition sensed along the low path with the location of the detected intensity transition sensed along the high path and determining whether the compared locations are positioned in a vertical relationship with one another. The contour tracing means of the apparatus is adapted to disregard license plate information below the baseline and above the top line when each of the compared locations is determined to be in a vertical relationship with respect to another of the compared locations and to disregard license plate information below the low path and above the high path when any of the the compared locations are determined not to be in a vertical relationship with any other compared location. Thus, when one compared location is determined not to be in a vertical relationship with any other compared location, at least a portion of the license plate region responsible for the intensity transition sensed along one of the high and low paths and at the one compared location is utilized in the identification of the license plate characters.
In a still further aspect of the present invention, the apparatus includes corner locating means associated with the edge locating means for identifying two left corners of the character set of the license plate wherein one of the left corners is contained along the defined baseline and the other of the left corners is contained along the defined top line and for identifying two right corners of the character set wherein one of the right corners is contained along the defined baseline and the other of the right corners is contained along the defined baseline. Joining means then joins the identified two left corners with a imaginary first line and joins the identified two right corners with an imaginary second line, and the contour tracing means for tracing the contours of the character edges is adapted to follow what would otherwise be a vertical edge of a character along a path which is generally parallel to one of the imaginary first and second lines.
In a yet further aspect of the apparatus of the present invention, the apparatus is adapted to read the characters of a license plate arranged in two superposed rows in the plate. In this connection, the apparatus includes a database containing criteria related to expected heights of license plates as a function of the length of the license plate character set and a scanner for scanning the license plate from one of the bottom and top of the plate toward the other of the bottom and top of the plate and through a first row of license plate characters. Measuring means in the apparatus measures the height and length of the first row of license plate characters scanned by the scanner, and comparing means compares the height of the first row of characters with the plate-height criteria of the database to determine whether the height of the first row of characters corresponds with the expected height of a license plate having a character set length equal to about the measured length of the first row of license plate characters. The scanner of the apparatus is adapted to continue scanning from the first row of license plate characters toward the other of the bottom and top of the plate for sensing the presence of intensity transitions associated with a second row of license plate characters when the comparing means determines that the height of the first row of characters does not correspond with an expected height of a license plate having a character set length equal to about the sensed length of the first row of license plate characters. The locating means of the apparatus is adapted to locate the edges of the characters in each row of characters found in the license plate as a function of the intensity transitions sensed in connection with each row, the tracing means is adapted to trace the contours of the located edges of the characters in each row, and the recognizing and identifying means is adapted to recognize and identify the characters in each row of license plate characters.
In one more aspect of the apparatus of the present invention, the apparatus includes state recognition means for sensing intensity transition patterns relating to decorative characteristics of the license plate and means for recognizing and identifying the state which issued the plate as a function of the sensed transition patterns.
In one more further aspect of the apparatus of the present invention, the apparatus includes state recognition means for sensing intensity transition patterns relating to non-decorative characteristics of the plate, which characteristics aid in the identification of the state which issued the plate, and means for recognizing and identifying the state which issued the license plate as a function of the sensed transition patterns.
In yet one more aspect of the apparatus of the present invention, the apparatus includes a database containing criteria relating to alphanumeric patterns of license plate character sets of plates issued by the identified state. The apparatus further includes means for comparing the alphanumeric pattern of the set of the identified characters to the alphanumeric patterns contained in the database and changing the identification of selected ones of the characters to conform the alphanumeric pattern of the set of identified characters to one conforming to an alphanumeric pattern of a character set issued by the identified state when the alphanumeric pattern of the set of the identified characters does not match an alphanumeric pattern contained in the database.
The method of the present invention includes the steps performed by the apparatus of the present invention. Accordingly, the method includes various aspects corresponding with the steps performed in accordance with the aforementioned aspects of the apparatus of the present invention.